In the paper manufacturing industry, a critical factor controlling the production of a cost effective paper product is the ability to rapidly and efficiently dry the paper stock. The plant must be able to cost effectively remove water in order to produce a paper product having a workable moisture content.
Upon entering the headbox of the papermaking plant, the paper stock is released onto a rapidly moving forming wire. Excess water filters through the forming wire and the paper fibers interlace with one another thereby forming a paper mat on the forming wire or screen. The formed paper web is carried on the screen through a series of rolls which smooth out the web and press out excess water.
At the end of the forming wire, the paper web is transferred to a dryer fabric which conveys the web around a series of steam heated dryer drums. Upon contact with the heated dryer drums, excess water in the web is evaporated which reduces the moisture content in the web to the desired level.
In order to reduce operating and capital costs, paper manufacturers have steadily increased production rates. During the standard drying process, the dryer fabric and web can travel at a speed of 4,000 feet per minute. Early drying schemes utilized a row of upper drying drums aligned above a row of lower drying drums and the web was conveyed around successive drying drums in a serpentine manner. As the web was transferred from an upper drum to a successive lower drum, it passed through a gap of open draw in which the web was not in contact with the dryer fabric. This open draw presented problems at high production speeds because the unsupported web often fluttered and sometimes broke, forcing paper production to stop.
In an attempt to minimize shutdowns, a single continuous flexible support dryer design was developed. The web was supported by a single flexible dryer fabric. Although this scheme used the same upper and lower rows of drying drums, it avoided the problems associated with the open draw by having the web remain in constant contact with the dryer fabric. While this new scheme reduced web fluttering and breakage, it created two different problems. First, due to the high web and dryer fabric speeds, the rapidly moving felt causes a high pressure area at the nip where the dryer fabric and web initially contact each drying drum. This localized high pressure, combined with inertial effects, causes the web to separate from the dryer fabric causing wrinkling and, in severe cases, breakage of the web. Second, by the nature of the configuration of the single continuous support dryer design, the dryer fabric was located between the lower dryer drum surfaces and the web. Because of its interior location, the dryer fabric insulates the web from the heat of the lower drying drums, thereby reducing drying efficiency for the lower drums.
Several configurations have been designed to solve the problems associated with the single dryer fabric continuous dryers. Known configurations include replacing lower dryer drums with unheated vacuum rolls, with unheated grooved rolls, or with unheated rolls having exterior vacuum sources. These configurations prevent pressure build-up at the nip where the dryer fabric and web contact the roll surface and save energy because they are not heated. However, because the configurations have only one level of drying drums, they occupy a large floor area of the paper manufacturing facility. This problem is intensified by the fact that single rolls, due to the small angle of web wrap on the dryers, are unable to utilize the maximum surface drying area of the upper drying drums. Also, the lack of open draw space and failure of these configurations to completely control air flow in the pocket above each roll minimizes the ability of utilizing air caps as a method of increasing drying efficiency. Finally, the cost of a vacuum roll, due to the high cost of drilling a multitude of vacuum draw holes in, is approximates the cost of a full-sized heated drying drum.
What is needed is an economical paper drying apparatus that maximizes the angle of web wrap on the dryer while completely controlling air flow in the pocket above each roll, and which eliminates disruptive localized pressure at the nips and allows the use of air caps in order to increase drying efficiency.